Exploring Rechargeable
Batteries by Peter Parker VK3YE
( first appeared in Amateur Radio, December 1999)

Rechargeable batteries: They're used
everywhere, and there's many different brands and types. Almost every
amateur has their own opinions on the merits of different types and the
best ways to look after them. Here we examine the main types available and
their suitability for various equipment amateurs use.

How rechargeable batteries work Batteries convert
stored chemical energy into electrical energy. This is achieved by causing
electrons to flow whenever there is a conductive path between the cell's
electrodes.

Electrons flow as a result of a chemical reaction
between the cell's two electrodes that are separated by an electrolyte.
The cell becomes exhausted when the active materials inside the cell are
depleted and the chemical reactions slow. The voltage provided by a cell
depends on the electrode material, their surface area and material between
the electrodes (electrolyte). Current flow stops when the connection
between the electrodes is removed.

Rechargeable cells operate on
the same principle, except that the chemical reaction that occurs is
reversed while charging. When connected to an appropriate charger, cells
convert electrical energy back into potential chemical energy. The process
is repeated every time the cell is discharged and recharged.

Different cells use different electrode materials and have
different voltage outputs (1.2, 1.5, 2 and 3.6 volts for the types
discussed here). Higher voltages are possible by connecting cells in
series. A set of several cells connected together is called a battery.
However, because lay people do not distinguish between a 1.5 volt cell and
a 9 volt battery (which comprises several cells), the term battery is
widely used for both batteries and cells.

The capacity of cells is
expressed in amp-hours (Ah) or milliamp-hours (mAh). The approximate time
that a battery will last per charge can be found by dividing the battery
pack capacity (normally written on the battery pack itself) by the average
current consumption of the device. Thus a 600 mAh battery pack can be
expected to power a receiver that takes 60mA for 10 hours.

Cells
can be visualised as consisting of a cell with a resistor in series. You
won't find an actual resistor should you split open a battery pack, but
the effect is the same. Some battery types have higher values of internal
resistance than others. High internal resistance doesn't matter if
powering items that draw fairly low currents (eg a clock or small
receiver). However, if running something like a 5-watt handheld
transceiver, a battery with a high internal resistance will not deliver
the current asked of it.Having explained
some of the characteristics important to all batteries, we will now look
at each cell type in turn.

Nickel-cadmium
(NiCad)Nickel-cadmium cells are the most commonly used
rechargeable batteries in consumer applications. They come in similar
sizes to non-rechargeable cells, so they can directly replace
non-rechargeable alkaline or carbon-zinc cells. NiCads have a lower
voltage output than non-rechargeable cells (1.2 vs 1.5 volts). This
difference is not important in most cases.

NiCads perform best between 16 and 26 degrees Celsius. Their
capacity is reduced at higher temperatures. Hydrogen
gas is created and there is a risk of explosion when cells are used below
0 degrees.

NiCad batteries have a low internal resistance.
This makes them good for equipment that draws large amounts of current (eg
portable transmitting gear). However low internal resistance means that
extremely high currents (as much as 30 amps for a C-sized cell!) will flow
if cells are short-circuited. Short-circuiting should be avoided as it can
cause heat build-up and cell damage.

Most portable transceivers
come with NiCad battery packs where the cells are welded to metal
connecting straps. There is good reason for this. In high-current
applications, the unknown (and varying) resistance between cells and
battery holder contacts can result in erratic operation. This is
especially so when the transceiver is used in a salt-laden environment. An
encased battery pack overcomes these difficulties and provides more
reliable operation.

The normal charging rate
is 10 per cent of a battery's capacity for 14 hours. For example,
if a battery pack has a 600 mAh rating, its correct charging current is 60
mA. Because the charging process is not 100% efficient, the charger needs
to be left running for about 14 hours instead of 10 hours. Higher charging
currents are possible, but the charging time needs to be proportionally
reduced. NiCads can be left on a trickle charger indefinitely if the
charging current is reduced to 2% of the battery's amp-hour rating. Avoid
the build up of heat during charging for long battery life.

NiCad
batteries require a constant current charger; ie one where the current
provided to the battery is fixed over the entire charging period. Such a
charger can be something as simple as an unregulated DC power supply with
a series resistor to limit the charging current into the cells. If the
charger's voltage and the battery's desired charging current is known,
Ohm's Law can be used to calculate the correct series resistor value.
Because NiCads have a low internal resistance, proper charging can occur
with several cells in series.

For best life, do not discharge
NiCads to less than 1.0 volt per cell. When charging, NiCads should read
1.45 volts per cell. If the cell voltage is higher during charging (eg 1.6
or 1.7 volts), the cell is faulty and should be discarded.

You'll
often hear discussions about the so-called 'memory effect' exhibited by
NiCad cells. This refers to the claimed tendency of cells not to deliver
their rated voltage when placed in a charger before being fully
discharged. Belief in the existence of the 'memory effect' is widespread
amongst users of NiCad batteries. However, textbooks and data from battery
manufacturers make little or no mention of it. Believers say that to
prevent it batteries must be discharged to 1 volt per cell before
charging. Non-believers say that this discharging merely reduces cell
life.

Evidence suggests that true 'memory effect' is rare. It was
first noticed in communications satellites where cells were discharged to
precisely the same discharge point every time. In casual amateur use
batteries are most unlikely to be discharged to the same point after every
use. Much of what is mistaken for the 'memory effect' is voltage
depression, which is caused by long, continuous overcharging, which causes
crystals to grow inside the cell. Fortunately both the 'memory effect' and
voltage depression can be overcome by subjecting the battery to one or
more deep charge/discharge cycles.

Another term you will hear is
'cell reversal'. This can occur when a battery of cells is discharged
below its safe 1.0 volt per cell. During this discharge, differences
between individual cells can lead to one cell becoming depleted before the
rest. When this happens, the current generated from the remaining active
cells will 'charge' the weakest cell, but in reverse polarity. This can
lead to the release of gas and permanent damage to the battery pack.

NiCads can short circuit due to the build up of crystals inside
the battery. The use of a fully-charged electrolytic capacitor placed
across the cell can effect a temporary cure. Over-discharging of batteries
invites short circuiting. Batteries should be stored charged. A lifespan
of 200 to 800 charges is typical for NiCad batteries.

Nickel metal hydride (NiMH)Like NiCads,
nickel-metal hydride cells provide 1.2 volts per cell. Battery makers
claim that NiMH cells do not suffer from the 'memory effect' and can be
recharged up to 1000 times.

NiMH cells are not as suitable as
NiCads for extreme current loads, but do offer a greater capacity in the
same cell size. A typical AA NiCad may have a 750 mAh, but a NiMH may
provide 1100 mAh - 45 percent more. This makes NiMH cells a good choice
for applications where long life is desired but current demands are not
high - eg portable receiving equipment.

NiCad chargers can be used
to charge NiMH batteries, but the charging time needs to be lengthened to
take NiMH's typically larger capacity into account. The main enemy of
rechargeable cells is heat. If cells get hot during charging, reduce the
charging current to no more than that recommended.

RAM cells are cheaper to buy
than NiCads. They can be recharged between 50 and 750 times. They also
have a greater capacity than do NiCads - 1500 mAh is typical for size AA
cells. RAM cells are good for use with outdoor and solar equipment as they
will work efficiently at temperatures up to and exceeding 60 degrees
Celsius.

RAM cells have a much higher internal resistance than
NiCads (0.2 ohms vs 0.02 ohms). This means that they cannot supply high
peak values of current. For this reason they are
unsuitable for use with standard amateur HTs. However, their high
capacity and long shelf life (5 years) makes them suitable for low powered
or emergency-use applications, such as clocks and emergency torches.

Chargers intended for NiCad and NiMH cells
will not charge rechargeable alkalines. This is because
rechargeable alkaline cells require a constant voltage source of between
1.62 and 1.68 volts to charge. RAM cells should be connected in parallel
rather than in series when charging several cells at a time. Unlike other
rechargeable batteries, RAM cells are pre-charged and do not require
charging before first use.

Lithium ionLithium ion cells are the most recent of the battery types discussed
here to come onto the market. They offer higher cell voltage (3.6 volts)
and greater capacity for a given volume. This makes them especially
suitable for handheld equipment where long operating times are important,
such as mobile phones.

As an example of what Lithium ion battery
packs can do, a typical lithium ion battery pack is 55x45x20mm but
provides 7.2 volts with a 1100 mAh capacity. Lithium ion batteries are
still quite expensive, but are coming into amateur use through their
inclusion in handheld transceivers such as Yaesu's VX-1R and VX-5R
models.DANGERS OF LITHIUM ION
BATTERIES! Important links below, off site!Litium
Battery Fires (Video
course on Litium Battery Safety) A must see!Litium
Battery Safety Concerns (off site)Litium Battery
(very complete discussion and information about them off
site)

Sealed lead acidSealed lead
acid batteries (or 'gel cells') are less popular than NiCads in handheld
equipment, but find widespread use as back up batteries in security
systems and for amateur portable operation. Per-cell voltage is 2.3 volts
when charged, and 1.8 volts when discharged. This equates to 13.8 and 10.8
volts respectively for a battery of six cells. For best use of the full
battery charge, equipment intended to operate with '12 volt' sealed lead
acid batteries should operate well (if not at full power) at voltages of
10.8 volts or less.

Gel cells are cheap, rugged and reliable and
should last several years at least. If you want a battery to run a QRP HF
station or a VHF/UHF handheld for several hours, they are the ideal
choice. They are also widely used with small solar systems.

Sealed
lead acid batteries can either be used on a cyclic charge regime (battery
connected to charger for a specific time) or continuous float use, where
the battery is across the charger any time it's not in use. Cyclic
chargers should charge at 2.4 or 2.5 volts per cell and be current limited
to prevent overcharge. In contrast continuous float charging (or trickle
charging) requires a charging voltage of only 2.3 volts per cell (13.8
volts for a '12 volt' battery). With both types of use the charger voltage
is held constant. Connect batteries in parallel if charging two or more
from the one charger.

Chargers for sealed lead acid batteries are
available commercially or can be made at home. Special gel cell charger
ICs exist to provide the necessary voltage and current regulation.
Alternatively chargers can be made from the more common regulator chips
such as the 723 or LM317. These chargers can be used to directly trickle
charge the smaller '12 volt' gel batteries. No damage is done if the
charger remains on, even when the battery is fully charged. This is
because as the battery voltage approaches 13.8, the charging current will
fall to negligible levels.

Sealed lead acid batteries should not
be charged at voltages higher than those indicated as safe above. This is
because high charging voltages (eg 2.6 volts per cell) will endanger the
battery due to the production of excess gas. At a 13.8 volt charging
voltage the production of gas is low, and the battery should give years of
service. Charging current should not exceed 20 per
cent of the rated amp hour capacity of cells. If using a high
current 13.8 volt power supply as a charger, some form of current limiting
is desirable to stay within the battery's limits.

Conclusion This article has examined the
characteristics of all major types of rechargeable batteries used by
amateurs. We learned that NiCads and sealed lead acid cells were best for
high current applications, while other varieties, such as rechargeable
alkaline and nickel metal hydride work well for low current applications.
The charging of batteries varies too - Rechargeable alkaline and sealed
lead acid required a constant voltage, but nickel cadmium and nickel metal
hydride cells needed a constant current to charge properly. In all cases
over-charging, through excessive voltages, currents or charging periods
can cause heating, gas build-up and possible cell damage. However, if you
treat your batteries well, you should have many years of successful
operation from them, whichever type you choose.

Acknowledgments

I wish to acknowledge the
people and organisations who have contributed to the writing of this
article. These include:

The late Bill Trenwith VK3ATW for
suggestions on the manuscript and imparting of knowledge gained through
many years as a mechanics teacher, model engineer and radio amateur.
Peter Wegner from Coorey & Co, distributors of BIG rechargeable
alkaline cells. Danielle Cvetkovic from Invensys Energy Systems Pty
Ltd for material on Hawker sealed lead acid batteries. Adeal Pty Ltd
for information on Varta's range of NiCad and NiMH cells.
References

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